The present invention relates, in general, to the field of oxidant heaters (air heaters and air pre-heaters) for use in coal-fired power plants and in particular to a system and method for use in oxy-fuel combustion which incorporates a novel regenerative oxidant preheater internal sector arrangement as well as the strategic positioning of the primary oxidant fan and primary oxidant mixer. The present invention relates in part to minimizing the loss of oxygen through leakage of oxidant into the gas side of a rotary regenerative oxidant preheater.
Air quality laws, both at the federal and state level have set increasingly stringent emission standards.
Often of particular concern are sulfur dioxide and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Acidic gases are known to be hazardous to the environment, such that their emission into the atmosphere is closely regulated by clean air statutes.
New technologies are addressing this problem so that fossil fuels and particularly coal can be utilized for future generations without polluting the atmosphere and contributing to global warming. One of the technologies being developed has potential for retrofit to existing pulverized coal plants, which are the backbone of power generation in many countries. This technology is oxy-fuel combustion which is the process of firing a fossil-fueled boiler with an oxygen-enriched gas mix instead of air. Almost all the nitrogen is removed from the input air, yielding a stream that is approximately 95 percent oxygen. Firing with pure oxygen would result in too high a flame temperature, so the mixture is diluted by mixing with recycled flue gas. Oxy-fuel combustion produces approximately 75 percent less flue gas than air fueled combustion.
About 70 percent to 80 percent of the flue gas exiting the wet scrubber of an oxy-fired pulverized coal combustion plant is returned to the boiler where oxygen is introduced to produce the combustion oxidant gas, while the remainder of the flue gas is sent to a purification and compression system where it is prepared to suit pipeline and storage requirements. Thus, it is imperative that the carbon dioxide concentration be as high as possible with a low concentration of sulfur, nitrogen, oxygen, and water as can be practically and economically achieved.
Oxy-fired pulverized coal combustion burns pulverized coal in an oxidant comprised of a mixture of relatively pure oxygen and recycled flue gas to reduce the net volume of flue gases generated from the combustion process in a boiler, and to substantially increase the concentration of carbon dioxide in the flue gases. The recycled flue gas represents a portion of the flue gases generated by the combustion process and acts to dilute the flame temperature and maintain the volume necessary to ensure adequate convective heat transfer to all boiler areas, and can also be used to dry and carry the pulverized coal to the combustion space of the boiler.
The oxidant used in oxy-fired pulverized coal combustion is, in one non-limiting instance, heated in rotary regenerative type air preheaters, even though such air preheaters encounter leakage from the air side to the gas side. Tubular and plate type air preheaters do not experience leakage and provide a reasonable alternative to the rotary regenerative air preheater at industrial boiler scale. However, this is not a cost effective alternative at the electric utility boiler scale.
In conventional pulverized coal firing, a small portion of the air required for combustion is used to dry and carry the pulverized coal to the burners for burning the coal in the furnace or combustion space of the boiler. This portion of the air is known as primary air. In direct firing systems, primary air is also used to dry the coal in the pulverizer. The remainder of the combustion air is introduced in a windbox housing the burners, and is known as secondary air.
Rotary regenerative air preheaters are relatively compact and are the most widely used type for combustion air preheating in electric utility boiler plants. Rotary regenerative air preheaters transfer heat indirectly by convection as a heat storage medium is periodically exposed to heat-emitting flue gases and heat-absorbing combustion air. The rotary regenerative air preheater includes a cylindrical shell or housing that contains a coaxial rotor packed with metal heat storing corrugated plates which are bundled so as to present flow passageways therebetween. The preheater is divided into a gas side which is under negative pressure and an air side which is under positive pressure. The most prevalent flow arrangement has the flue gases entering the top of the rotor and the combustion air entering the bottom of the rotor in counter flow fashion. Consequently, the cold air inlet and the cooled gas outlet are at one end of the preheater, usually referred to as the cold end, the hot gas inlet and the heated air outlet are at the opposite end of the preheater, usually referred to as the hot end. As a result, an axial temperature gradient exists from the hot end of the rotor to the cold end of the rotor. In response to this temperature gradient, the rotor tends to distort and to assume a shape similar to that of an inverted dish, commonly referred to as rotor turndown.
In operation, the rotor is rotated slowly about a central shaft, making one to three revolutions per minute causing each bundle of heat absorbing plates to be placed, alternately, into the flow path of the heat-emitting flue gases and the flow path of the heat-absorbing combustion air. The most notable characteristic of rotary regenerative air preheaters is that a small but significant amount of air leaks from the positive pressure air side to the negative pressure gas side due to rotor turndown and the rotary operation of the air preheater. In order to prevent undue leakage from the air side to the gas side, the air preheater is provided with radial, axial and peripheral seals. It is known to construct these seals of thin, flexible metal. The seals are adjusted when the gaps are the largest. This means that, when the gaps are small due to expansion of the rotor and the housing, the seals may be severely bent and forced into high contact pressure with the rotor or housing. For this reason, seals wear relatively quickly and require replacement.
In a prior art or conventional regenerative air or oxidant preheater arrangement, the primary air or oxidant is at a positive pressure of about 40 inches of water gage (“inches wg”), the secondary air or oxidant is at a positive pressure of about 20 inches wg, and the flue gas is at a negative pressure of about 5 inches wg. This conventional air or oxidant preheater has the air or oxidant side of the preheater divided into three sectors, a central sector which receives the primary air or oxidant and is flanked by a pair of sectors which receive the secondary air or oxidant and are located adjacent the flue gas side portion of the preheater. This arrangement minimizes the pressure difference across the seals between the air or oxidant side and the gas side to about 25 inches wg, which results in 7 percent to 14 percent leakage of air or oxidant into the flue gas. These values, though representative of a coal fired plant, may vary depending on fuel and equipment variations and are not intended as absolute.
In an oxy-fired pulverized coal plant the combustion process is carried out by the oxidant, which is comprised of a mixture of relatively pure oxygen and recycled flue gas, with a portion thereof being used to dry and transport the pulverized coal to the burners and the remainder being introduced into the boiler combustion space. The oxidant must be heated before entering the combustion process, and the equipment of choice is a rotary regenerative air preheater since it is cost effective for electric utility power plants. However, the leakage occurring in the regenerative oxidant preheater from the positive pressure oxidant to the negative pressure flue gas represents a loss of oxygen and recycled flue gas to the gas side of the regenerative oxidant preheater. This loss of oxygen along with the recycle gas requires additional oxygen production in an air separation unit to make up for the loss of oxygen, and it also requires the removal of the leaked oxygen from the product gas in a compression and purification unit before the concentrated carbon dioxide can be disposed of via storage or use for enhanced oil recovery, since pipeline line and use constraints require that the flue gas be as high in concentration of carbon dioxide and as low in concentration of nitrogen, sulfur, oxygen and water, as practical. Both of these remedial procedures result in increased plant operating costs. Thus, oxidant introduction into the flue gas must be minimized or eliminated. Furthermore, it is undesirable for an oxidant with a high concentration of oxygen to be exposed to ash potentially containing some combustible carbon and thereby raising the concern of fire.
In one process variation, warm recycle, the flue gas leaving the oxidant preheater is immediately split into two streams. One stream passes through particulate, SO2, and moisture removal as described herein, before being further split between the primary and exit stream to a CPU. The other, secondary stream, passes through particulate removal, and is routed back to the oxidant preheater in a “warm” state (about 400° F.).
Regenerative oxidant heaters transfer heat indirectly by convection as a heat storage medium is periodically rotated into the hotter and cooler flow streams. In steam generating plants, tightly packed bundles of corrugated steel plates serve as the storage medium. In these units either the steel plates, or surface elements, rotate through oxidant (or air) and gas streams, or rotating ducts direct oxidant and gas streams through stationary surface elements.
The conventional regenerative oxidant preheater which is most commonly used is the Ljungstrom® type which features a cylindrical shell plus a rotor which is packed with bundles of heating surface elements which are rotated through counterflowing oxidant and gas streams. FIG. 1 shows the typical positioning of the sectors through which the primary oxidant stream 58 and secondary oxidant stream 34 as well as the counterflowing flue gas stream 98 flows in a conventional Ljungstrom® type regenerative oxidant heater. As is shown, oxidant flows through one half of the rotor and flue gas which comes from the boiler's gas exit flows through the other half. Additionally, the oxidant side (or “air side”) consists of two sectors, one for the primary stream and one for the secondary stream.
Another common conventional regenerative oxidant preheater sector arrangement includes that which is disclosed in United States Patent Application Publication No. 2006/0090468. The regenerative oxidant preheater of United States Patent Application Publication No. 2006/0090468 is adapted to receive a flow of cool oxidant in counterflow to the flow of hot flue gas and to provide a heat exchange between the cool oxidant and the hot flue gas to convert the cool oxidant into the heated combustion oxidant. FIG. 2 is a bottom cross sectional view of the rotor of an oxidant preheater with a typical sector arrangement, which is also employed in United States Patent Application Publication No. 2006/0090468. It shows the primary oxidant sector 40, and the secondary oxidant sector 42, through which respective primary and secondary oxidant streams flow toward a boiler. The primary and secondary sectors 40 and 42, respectively, are separated by a sector plate 44 and they are both adjacent to flue gas sector 38, through which flue gas flows away from a boiler. Additionally, the primary and secondary sectors 40 and 42 are separated from the flue gas sector 38 by sector plates 46 and 48.
In conventional regenerative oxidant heaters for an air-fired power plant, a typical pressure for the primary oxidant stream is high as compared with that of both the secondary air stream and the flue gas stream. For example, a typical pressure for the primary oxidant stream is about +40 inches wg, for the secondary oxidant stream it is about +20 inches wg, and for the flue gas stream from the boiler it is about −5 inches wg. Thus, between the secondary air sector and gas side there is about a +25 inches water gauge difference in air pressure which can result in 14 percent leakage of the oxidant into the gas side. Furthermore, the pressure gradient between the primary air sector and the gas side is much greater.
In all regenerative heaters which use conventional sector arrangements such as those discussed above, the rotating heat exchanger in combination with the pressure differential between the respective streams causes inherent leakage between the air (or oxidant) side and the gas side conveying the hot flue gas from the boiler.
However, in oxy-fuel combustion, all these configurations result in leakage from the oxidant to the gas side. In addition most of the oxygen must be added to the recycled flue gas upstream of the oxygen preheater in order to achieve a reasonable flue gas temperature leaving the oxidant heater. Thus any leakage results in the loss of costly oxygen along with recycle gas to the exit stream thereby necessitating additional oxygen production in an air separation unit (ASU) to make up for the loss as well as removal of additional oxygen from the product gas in a compression and purification unit (CPU) before the concentrated CO2 can be disposed of. A need exists to minimize such leakage as the cost of additional oxygen production and removal is prohibitive.
An additional problem encountered when using an oxidant preheater employing a conventional sector arrangement in oxy-fuel combustion, is difficulty achieving acceptable exit gas temperature. Due to the high temperatures of the flue gas and oxidant streams, it can be very difficult, especially evident in warm recycle, to achieve an acceptable gas temperature at the oxidant preheater exit.
Furthermore, the concern about costly oxygen loss as well as the problem caused by the high temperature of the oxidant(s) and flue gas makes it difficult to achieve a design that balances both acceptable heat exchanges within the oxidant preheater as well as reasonable oxidant preheater exit gas temperatures.
When attempting to achieve such a balance, a typical known step is to add cool oxidant to the oxidant flow stream(s) prior to (i.e., upstream from) the oxidant heater. However, this conventional method is undesirable because leakage from the oxygenated oxidant stream(s) into the boiler flue gas stream results in the loss of costly oxygen. The loss of oxygen occurs because a substantial portion of it will flow in the stream which leads to the compression process. Additionally, if the oxygen is added to the primary or secondary oxidant stream after the oxidant preheater an acceptable oxidant preheater design becomes more difficult to achieve.
Another common approach which has been taken in an attempt to reduce leakage and hence loss of costly oxygen, is to locate the primary oxidant fan downstream of the oxidant heater. However, this approach has consistently been ineffective.
Additionally, a known alternative to using a regenerative oxidant preheater in order to eliminate internal leakage and avoid loss of costly oxygen, is to use expensive separate primary and secondary tubular or plate type preheaters for the secondary and primary oxidant streams which completely separates the oxidant side and the gas side allowing no leakage. However, although this alternative may be reasonable for use on the industrial boiler scale in air fired applications, it is not cost effective when applied to large utility boilers. Additionally, separate tubular preheaters require considerably more space than regenerative oxidant heaters and tubular preheaters are susceptible to significant internal leakage with age, thus inevitably resulting in oxidant loss.
Accordingly there is a clear need for a cost effective system and method which incorporates a regenerative oxidant preheater design that will minimizes the loss of costly oxygen normally caused by internal oxidant preheater leakage and which will also achieve an acceptable balance between reasonable heat exchange within the oxidant preheater and a reasonable oxidant preheater exit flue gas temperature.